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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/101460] Useless cascade of overload resolution errors for
 invalid expression
Date: Thu, 15 Jul 2021 11:58:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101460

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-07-15
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I suppose the problem here is that the failed constant evaluation of frob<-=
1>()
only happens as a result of trying to use it as a template argument (where a
constant expression is required) which doesn't happen until overload resolu=
tion
is performed. So we have to at least being doing OR before we get a problem.

But maybe we can remember that it failed once, and so stop trying. If it's =
not
a valid constant expression, *that* is the error, not the fact that none of=
 the
get<N> overloads can be called with an invalid constant expression.

Even if I change the code to do this it keeps trying overload resolution for
get<n>:

template<int N> void get_n(tuple& t) {
  constexpr unsigned n =3D frob<N>();
  get<n>(t);
}

The constexpr initialization of 'n' failed with a static_assert *and* a
non-constant narrowing conversion, so why do we continue and even attempt
overload resolution for 'get'?=